Why spiders hang with their heads down
Katara asked me last week why spiders hang in their webs with their
heads downwards, and I said I would try to find out. After a cursory
Google search, I was none the wiser, so I tried asking the Wikipedia
"reference desk" page. I did not learn anything useful about the
spiders, but I did learn that the reference desk page is full of
people who know even less about spiders than I do who are nevertheless
willing to post idle speculations.

Fortunately, I was at a meeting this week in Durham that was also
attended by three of the world's foremost spider experts. I put the
question to Jonathan
A. Coddington, curator of arachnids for the Smithsonian
Institution.

Professor Coddington told me that it was because the spider prefers
(for obvious mechanical and dynamic reasons) to attack its prey from
above, and so it waits
the upper part of the web and constructs the web so that the principal
prey-catching portion is below. When prey is caught in the web, the
spider charges down and attacks it.

I had mistakenly thought that spiders in orb webs (which are the
circular webs you imagine when you try to think of the canonical
spiderweb) perched in the center. But it is only the topological
center, and geometrically it is above the midline, as the adjacent
picture should make clear. Note that more of the radial threads are
below the center than are above it.

Homosexuality is not hereditary
A just read a big pile of blog comments that all said that
homosexuality couldn't be hereditary, because if it were, natural
selection would have gotten rid of it by now.

But natural selection is more interesting than that. This article
will ignore the obvious notion of homosexuals who breed anyway. Here
is one way in which homosexuality could be entirely hereditary and
still be favored by natural selection.

Suppose that human sexuality is extremely complicated, which should
not be controversial. Suppose, just for concreteness, that there are
137 different genes that can affect whether an individual turns out
heterosexual or homosexual. Say that each of these can either be
either in state Q or state S, and that and that any individual will
turn out homosexual if any 93 of the 137 genes are in state Q,
heterosexual otherwise.

The over-simplistic argument from natural selection says that the Q
states will be bred out of the population, and that S will be
increasingly predominant over time.

Now let's consider an individual, X, whose family members tend
to carry a lot of Q genes.

Suppose X's parents have a lot of Q genes, around 87 or 90.
X's parents' siblings, who resemble them, will also have a lot
of Q genes, and have a high probability of being homosexual. Having
no children of their own, they may contribute to X's welfare,
maybe by caring for X or by finding food for X.

In short, for every gay uncle X has, that is one additional set
of cousins with whom X does not have to compete for
scarce resources.

This could well turn out to be a survival advantage for X over
someone from a family of people without a lot of Q genes, someone who
is competing for food with a passel of cousins, none of whom
ever really get enough to eat, someone whose aunt might even try to kill
them in order to benefit her own children.

Perhaps X turns out to be homosexual and never breeds, but
X probably has some siblings, in which case X might be
an advantageous gay uncle or lesbian aunt to one of his or her own
nieces or nephews, who, remember, are carrying a lot of the same
genes, including the Q genes.

It might not actually work this way, of course, and in most ways it
probably doesn't. The only point here is to show that natural
selection does not necessarily rule out the idea of inherited
homosexuality; people who think it must, have not exercised enough
imagination.

(Now that I have finished writing this article, it occurs to me that
the same argument applies to bees and ants; most individuals in a bee
or ant colony are sterile. Who would be foolish enough to argue that
this trait will soon be bred out of the colony?)

Time and time again, biologists baffled by some apparently futile or
maladroit bit of bad design in nature have eventually come to see that
they have underestimated the ingenuity, the sheer brilliance, the
depth of insight to be disovered in one of Mother Nature's creations.
Francis Crick has mischievously baptized this trend in the name of his
colleague Leslie Orgel, speaking of what he calls "Orgels Second Rule:
Evolution is cleverer than you are."

Back in the 1950's
and 1960's, James V. McConnell at the University of Michigan was
doing some really interesting work on learning and memory in planaria
flatworms, shown at right. They used to publish their papers in their
own private journal of flatworm science, The Journal of
Biological Psychology. They didn't have enough material for a
full journal, so when you were done reading the Journal,
you could flip it over and read the back half, which was a
planaria-themed humor magazine called The Worm-Runner's
Digest. I swear I'm not making this up.

(I think it's time to revive the planaria-themed humor magazine.
Planaria are funny even when they aren't doing anything in
particular. Look at those googly eyes!)

(For some reason I've always found planaria fascinating, and I've
known about them from an early age. We would occasionally visit my
cousin in Oradell, who had a stuffed toy which was probably intended
to be a snake, but which I invariably identified as a flatworm.
"We're going to visit your Uncle Ronnie," my parents would say, and I
would reply. "Can I play with Susan's flatworm?")

Anyway, to get on with the point of this article, McConnell made the
astonishing discovery that memory has an identifiable chemical basis.
He trained flatworms to run mazes, and noted how long it took to do
so. (The mazes were extremely simple T shapes. The planarian goes in
the bottom foot of the T. Food goes in one of the top arms,
always the same one. Untrained planaria swim up the T and then turn
one way or the other at random; trained planaria know to head toward
the arm where the food always is. Pretty impressive, for a worm.)

Then McConnell took the trained worms and ground them up and fed them
to untrained worms. The untrained worms learned to run the maze a lot
faster than the original worms had, apparently demonstrating that
there was some sort of information in the trained worms that survived
being ground up and ingested. The hypothesis was that the information
was somehow encoded in RNA molecules, and could be physically
transferred from one individual to another. Isn't that a wonderful
dream?

You can still see echoes of this in the science fiction of the era.
For example, a recurring theme in Larry Niven's early work is "memory
RNA", people getting learning injections, and pills that impart
knowledge when you swallow them. See World Out of Time
and The Fourth Profession, for example. And I once had a
dream that I taught a giant planarian to speak Chinese, then fried it in
cornmeal and ate it, after which I was able to speak Chinese. So when
I say it's a wonderful dream, I'm speaking both figuratively and
literally.

Unfortunately, later scientists were not able to reproduce McConnell's
findings, and the "memory RNA" theory has been discredited. How to
explain the cannibal flatworms' improved learning times, then? It
seems to have been sloppy experimental technique. The original
flatworms left some sort of chemical trail in the mazes, that
remained after they had been ground up. McConnell's team didn't wash
the mazes in between tests, and the cannibal flatworms were able to
follow the trails later; it had nothing to do with their diet. Bummer.

So a couple of years ago I was poking around, looking for more
information about this, and in particular for a copy of McConnell's
famous paper Memory transfer through cannibalism in
planarium, which I didn't find. But I did find the
totally unrelated Robert French paper. It mentions McConnell, as an
example of another cool-sounding and widely-reported theory that took
a long time to dislodge, because it's hard to produce clear evidence
that cannibal flatworms aren't in fact learning from their
lunch meat, and because the theory that they aren't learning is so much less
interesting-sounding than the theory that they are. News outlets
reported a lot about the memory RNA breakthrough, and much less about
the later discrediting of the theory.

French's paper, you will recall, refutes the interesting-sounding
hypothesis that infants resemble their fathers more strongly then they
do their mothers, and has many of the same difficulties.

There are counterexamples. Everyone seems to have heard that the
Fleischmann and Pons tabletop cold fusion experiment was an error.
And the Hwang Woo-Suk stem cell fraud is all over the news these
days.

Do infants resemble their fathers more than their mothers?
Back in 1995, Christenfeld and Hill published a paper claimed to have
found evidence that infants tended to resemble their fathers more than
they resembled their mothers. The evolutionary explanation for this,
it was claimed, is that children who resemble their fathers are less
likely to be abandoned by them, because their paternity would be less
likely to be doubted. The pop science press got hold of
it—several years later, as they often do—and it was widely
reported for a while. Perhaps you heard about it.

A couple years ago, while looking for something entirely unrelated, I
ran across the paper of French et al. titled The Resemblance of
One-year-old Infants to Their Fathers: Refuting Christenfeld &
Hill. French and his colleagues had tried to reproduce
Christenfeld and Hill's results, with little success; they suggested
that the conclusion was false, and offered a number of arguments as to
why the purported resemblance should not exist. Of course, the
pop science press was totally uninterested.

At the time, I thought, "Wow, I wish I had a way to get a lot of
people to read this paper." Then last month I realized that my
widely-read blog is just the place to do this.

Before I go on, here
is the paper. I recommend it; it's good reading, and only six
pages long. Here's the abstract:

In 1995 Christenfeld and Hill published a paper that purported to show
at one year of age, infants resemble their fathers more than their
mothers. Evolution, they argued, would have produced this result since
it would ensure male parental resources, since the paternity of the
infant would no longer be in doubt. We believe this result is
false. We present the results of two experiments (and mention a third)
which are very far from replicating Christenfeld and Hill's data. In
addition, we provide an evolutionary explanation as to why evolution
would not have favored the result reported by Christenfeld and Hill.

In the first study done by French, participants were presented with a
1-, 3-, or 5-year-old child's face, and the faces of either the father
and two unrelated men, or the mother and two unrelated women. The
participants were invited to identify the child's parent. They did
indeed succeed in identifying the children's parents somewhat more
often than would have been obtained by chance alone. But the
participants did not identify fathers more reliably than they
identified mothers.

The second study was similar, but used only 1-year-old infants. (The
Christenfeld and Hill claim is that one year is the age at which
children most resemble their fathers.)

French points out that although the argument from evolutionary
considerations is initially attractive, it starts to disintegrate when
looked at more closely. The idea is that if a child resembles its
father, the father is less likely to doubt his paternity, and so is
less likely to withhold resources from the child. So there might be a
selection pressure in favor of resembling one's father.

But now turn this around: if a father can be sure of paternity because
the children look like him, then he can also be sure when the children
aren't his because they don't resemble him. This will
create a very strong selection pressure in favor of children
resembling their fathers. And the tendency to resemble one's father
will create a positive feedback loop: the more likely kids are to look
like their fathers, the more likely that children who don't resemble
their fathers will be abandoned, neglected, abused, or killed. So if
there is a tendency for infants to resemble their fathers more than
their mothers, one would expect it to be magnified over time, and to
be fairly large by now. But none of the studies (including the
original Christenfeld and Hill one) found a strong tendency for
children to resemble their fathers.

But, as French notes, it's hard to get people to pay attention to a
negative result, to a paper that says that something interesting
isn't happening.

The example given in the article that I found most interesting was
"Why don't we just have one ear in the middle of our face?". As I
said earlier, I think the mark of a good question is that it's quick
to ask and long to answer. I've been thinking about this one for
several days now, and seems pretty long to answer.

Any reasonable answer to this question is going to be based on
evolutionary and adaptive considerations, I think. When you answer
from evolutionary considerations, there are only a few kinds of
answers you can give:

It's that way because it confers a survival or reproductive advantage.

It's that way because that's the only way it can be made to work.

It's that way because it doesn't really matter, and that's just
the way it happened to come out.

All of these, I think, have a role to play here. Having two ears is
useful for redundancy: if you lose one, you can still hear, so there
is a survival advantage to having two ears, just as there is for
having two eyes and two kidneys. Why two eyes? In case you lose
one. Why two kidneys? In case one fails. Why two nostrils? So you
can still breathe even when one is clogged.

(Why only one heart? There's no benefit to having two; if you lose
50% of your cardiac capacity, you'll die anyway. Why one mouth? It
needs to be big enough to eat with, and anyway, you can't lose it.
Why one liver? No reason; that's just the way it's made; two livers
would work just as well as one. Why two lungs? I'm not sure; I
suppose it's a combination between "no reason, that's the way it's
made" (#3 above) and "because that way you can still breathe even if
one lung gets clogged up" (#1).)

The positioning of your ears is important. Having two ears far apart
on the sides of your head allows you to locate sounds by triangulation.
Triangulation requires at least two ears, and requires that they be as
far apart as possible. This also explains why the ears are on the
sides rather than the front.

Consider what would go wrong if the positions of the eyes and ears
were switched. The ears would be pointed in the same direction, which
would impede the triangulation-by-sound process. The eyes would be
pointed in opposite directions, which would completely ruin the
triangulation-by-sight process; you would completely lose your depth
perception. So the differing position of the eyes and ears can be
seen a response to the differing physical properties of light and
sound: light travels in straight lines; sound does not.

The countervailing benefit to losing your depth perception would be
that you would be able to see almost 180 degrees around you. Many
animals do have their eyes on the side of their heads: antelopes,
rabbits, and so forth. Prey, in other words. Predators have eyes on
the fronts of their heads so that they can see the prey they are
sneaking up on. Prey have eyes on the sides of their heads so that
predators can't sneak up on their flanks. Congratulations: you're
predator, not prey.

Animals do have exactly one nose in the middle of their face.
Why not two? Here, triangulation is not an issue at all. Having one
nose on each side of your head would not help you at all to locate the
source of an odor. So the nose is stuck in the middle of the head, I
suppose for mostly mechanical reasons: animals with noses evolved from
animals with a long breathing tube down the middle of their bodies.
The nose arises as sensors stuck in the end of the tube. This is
another explanation for the one mouth.

Another consideration is symmetry. The body is symmetric, so if you
want two ears, you have to put one on each side. Why is this?
I used to argue that it was to save information space in the genome:
there is only so much room in your chromosomes for instructions about
how to build your body, so the information must be compressed. One
excellent way to compress it is to make some parts like other parts
and then express the differences as diffs. This, I used to say, is
why the body is symmetric, why your feet look like your hands, and why
men's and women's bodies are approximately the same.

I now think this is wrong. Well, wrong and right, essentially right,
but mostly wrong. The fact is, there is plenty of space in the
chromosomes for instructions about all sorts of stuff. Chromosomes
are really big, and full of redundancy and junk. And if it's so
important to save space in the chromosome, why is the inside of your
body so very asymmetric?

I now think the reason for symmetries and homologies between body
parts is less to do with data compression and storage space in the
chromosome, and more to do with the shortness of the distance between
points in information space. Suppose you are an animal with two
limbs, each of which has a hand on the end. Then a freak mutation
occurs so that your descendants now have four limbs. The four limbs
will all have similar hands, because mutation cannot invent an
entirely new kind of hand out of thin air. Your genome contains only
one set of instructions for appendages that go on the ends of limbs,
so these are the instructions that are available to your descendants.
These instructions can be duplicated and modified, but again, there is
no natural process by which a new set of instructions for a new kind
of appendage can be invented from whole cloth. So your descendants'
hands will look something like their feet for quite a long time.

Similarly, there is a certain probability, say p, of an
earless species evolving something that functions as an ear. The
number p is small, and ears arise only because of natural
selection in favor of having ears. The chance that the species will
simultaneously and independently evolve two completely different kinds
of ear structures is no more than p2, which is
vanishingly small. And once the species has something earlike, the
selection pressure in favor of the second sort of ear is absent. So a
species gets one kind of ear. If having two ears is beneficial, it is
extremely unlikely to arise through independent evolution, and much
more likely to arise through a much smaller mutation that directs the
same structure, the one for which complete instructions already exist
in the genome, to appear on each side of the head.

So this is the reason for bodily symmetry. Think of (A) an earless
organism, (B) an organism with two completely different ears, and (C)
an organism with two identical ears. Think of these as three points
in the space of all possible organisms. The path from point A to C is
both much shorter than the path from A to B, and also much more likely
to be supported by selection processes.

Now, why is the outside of the body symmetric while the inside is not?
I haven't finished thinking this through yet. But I think it's
because the outside interacts with the gross physical world to a much
greater extent than the inside, and symmetry confers an advantage in
large-scale physical interactions. Consider your legs, for example.
They are approximately the same length. This is important for
walking. If you had a choice between having both legs shortened six
inches each, and having one leg shortened by six inches, you would
certainly choose the former. (Unless you were a sidehill winder.)
Similarly, having two different ears would mess up your hearing,
particularly your ability to locate sounds. On the other hand,
suppose one of your kidneys were much larger than the other. Big
deal. Or suppose you had one giant liver on your right side and none
on the left. So what? As long as your body is generally balanced, it
is not going to matter, because the liver's interactions with the
world are mostly on a chemical level.

So I think that's why you have an ear on each side, instead of one ear
in the middle of your head: first, it wouldn't work as well to have
one. Second, symmetry is favored by natural selection for
information-conserving reasons.

I get a new job
Where did my blog go for the past six weeks? Well, I was busy with
another project. Usually, when I am busy with a project, it shows up
here, because I am thinking about it, and I want to write about what I
am thinking. As Hans Arp said, it grows out of me and I keep cutting
it off, like toenails. But in this case I could not write about the
project here, because it was a secret. I was looking for a new job,
and I did not want my old job to find out before I was ready.

(Many people have been surprised to learn that I have a job;
they remember that for many years I was intermittently a software
consultant and itinerant programming trainer. But since January 2004
I have been regularly employed to do maintenance programming for the
University of Pennsylvania's Networking and Telecommunications
group.)

Anyway, the job hunt has come to a close. I accepted a new job, put
in my resignation letters at the old one, and can stop thinking about
it for a while. The new work will be head software engineer at the
Penn Genomics Institute. I will try to develop software for genetic
biologists to use in their research. I expect that the new job will
suit me somewhat better than the old one. I like that it is connected
to science, and that I will be working with scientists. The work
itself is important; genomics is going to change everything in the
world. Also, it pays rather more than the old one, although that was
not the principal concern.

So with any luck blog posts will resume here, and eventually some
genomics-related articles may start appearing.

The octopus and the creation of the cosmosIn
an earlier post, I mentioned the lucky finds you sometimes make
when you're wandering at random in a library. Here's another such.
In 2001 I was in Boston with my wife, who was attending the United
States Figure Skating Championships. Instead of attending the Junior
Dance Compulsories, I went to the Boston Public Library, where I
serendipitously unearthed the following treasure:

Although we have the source of all things from chaos, it is a
chaos which is simply the wreck and ruin of an earlier
world....The drama of creation, according to The Hawaiian
account, is divided into a series of stages, and in the very
first of these life springs from the shadowy abyss and dark
night...At first the lowly zoophytes and corals come into
being, and these are followed by worms and shellfish, each
type being declared to conquer and destroy its predecessor, a
struggle for existence in which the strongest survive....As
type follows type, the accumulating slime of their decay
raises land above the waters, in which, as spectator of all,
swims the octopus, the lone survivor of an earlier world.

Morphogenetic puzzles
In a recent
post, I briefly discussed puzzling issues of morphogenesis: when a
caterpillar pupates, how do its cells know how to reorganize into a
butterfly? When the blastocyst grows inside a mammal, how do its
cells know what shape to take? I said it was all a big mystery.

A reader, who goes by the name of Omar, wrote to remind me of the
"Hox" (short for "homeobox") genes discussed by Richard Dawkins in
The Ancestor's Tale. (No "buy this" link; I only do that
for books I've actually read and recommend.) These genes are
certainly part of the story, just not the part I was wondering
about.

The Hox genes seem to be the master controls for notifying developing
cells of their body locations. The proteins they manufacture bind
with DNA and enable or disable other genes, which in turn manufacture
proteins that enable still other genes, and so on. A mutation to the
Hox genes, therefore, results in a major change to the animal's body
plan. Inserting an additional copy of a Hox gene into an invertebrate
can cause its offspring to have duplicated body segements; transposing
the order of the genes can mix up the segments. One such mutation,
occurring in fruit flies, is called antennapedia, and causes the flies' antennae to be
replaced by fully-formed legs!

So it's clear that these genes play an important part in the overall
body layout.

But the question I'm most interested in right now is how the
small details are implemented. That's why I specifically
brought up the example of a ring finger.

Or consider that part of the ring finger turns into a fingernail bed
and the rest doesn't. The nail bed is distally located, but the
most distal part of the finger nevertheless decides not
to be a nail bed. And the ventral part of the finger at the same
distance also decides not to be a nail bed.

Meanwhile, the ear is growing into a very complicated but specific
shape with a helix and an antihelix and a tragus and an antitragus.
How does that happen? How do the growing parts communicate between
each other so as to produce that exact shape? (Sometimes, of course,
they get confused; look up accessory
tragus for example.)

In computer science there are a series of related problems called
"firing squad problems". In the basic problem, you have a line of
soldiers. You can communicate with the guy at one end, and other than
that each soldier can only communicate with the two standing next to
him. The idea is to give the soldiers a protocol that allows them to
synchronize so that they all fire their guns simultaneously.

It seems to me that the embryonic cells have a much more difficult
problem of the same type. Now you need the soldiers to get into an
extremely elaborate formation, even though each soldier can only see
and talk to the soldiers next to him.

Omar suggested that the Hox genes contain the answer to how the fetal
cells "know" whether to be a finger and not a kneecap. But I think
that's the wrong way to look at the problem, and one that glosses over
the part I find so interesting. No cell "becomes a finger". There is
no such thing as a "finger cell". Some cells turn into hair follicles
and some turn into bone and some turn into nail bed and some turn into
nerves and some turn into oil glands and some turn into fat, and yet
you somehow end up with all the cells in the right places turning into
the right things so that you have a finger! And the finger has hair
on the first knuckle but not the second. How do the cells know which
knuckle they are part of? At the end of the finger, the oil glands
are in the grooves and not on the ridges. How do the cells know
whether they will be at the ridges or the grooves? And the fat pad is
on the underside of the distal knuckle and not all spread around. How
do the cells know that they are in the middle of the ventral surface
of the distal knuckle, but not too close to the surface?

Somehow the fat pad arises in just the right place, and decides to
stop growing when it gets big enough. The hair cells arise only on
the dorsal side and the oil glands only on the ventral side.

How do they know all these things? How does the cell decide that it's
in the right place to differentiate into an oil gland cell? How does
the skin decide to grow in that funny pattern of ridges and grooves?
And having decided that, how do the skin cells know whether they're
positioned at the appropriate place for a ridge or a groove? Is there
a master control that tells all the cells everything at once? I bet
not; I imagine that the cells conduct chemical arguments with their
neighbors about who will do which job.

One example of this kind of communication is phyllotaxis, the way
plants decide how to distribute their leaves around the stem. Under
certain simple assumptions, there is an optimal way to do this: you
want to go around the stem, putting each leaf about 360°/φ
farther than the previous one, where φ is ½(1+√5).
(More about this in some future post.) And in fact many plants do
grow in just this pattern. How does the plant do such an elaborate
calculation? It turns out to be simple: Suppose leafing is controlled
by the buildup of some chemical, and a leaf comes out when the
chemical concentration is high. But when a leaf comes out, it also
depletes the concentration of the chemical in its vicinity, so that
the next leaf is more likely to come out somewhere else. Then the
plant does in fact get leaves with very close to optimal placement.
Each leaf, when it comes out, warns the nearby cells not to turn into
a leaf themselves---not until the rest of the stem is full, anyway. I
imagine that the shape of the ear is constructed through a more
complicated control system of the same sort.

In an
earlier post I remarked that "The liver of arctic animals . . .
has a toxically high concentration of vitamin D". Dennis Taylor has
pointed out that this is mistaken; I meant to say "vitamin A".
Thanks, Dennis.

B and C vitamins are not toxic in large doses; they are water-soluble
so that excess quantities are easily excreted. Vitamins A and D are
not water-soluble, so excess quantities are harder to get rid of.
Apparently, though, the liver is capable of storing very large
quantities of vitamin D, so that vitamin D poisoning is extremely
rare.

The only cases of vitamin A poisoning I've heard of concerned either
people who ate the livers of polar bears, walruses, sled dogs, or
other arctic animals, or else health food nuts who consumed enormous
quantities of pure vitamin A in a misguided effort to prove how
healthy it is. In On Food and Cooking, Harold McGee writes:

In the space of 10 days in February of 1974, an English health food
enthusiast named Basil Brown took about 10,000 times the recommended
requirement of vitamin A, and drank about 10 gallons of carrot juice,
whose pigment is a precursor of vitamin A. At the end of those ten
days, he was dead of severe liver damage. His skin was bright
yellow.

(First edition, p. 536.)

There was a period in my life in which I was eating very large
quantities of carrots. (Not for any policy reason; just because I
like carrots.) I started to worry that I might hurt myself, so I did
a little research. The carrots themselves don't contain vitamin A;
they contain beta-carotene, which the body converts internally to
vitamin A. The beta-carotene itself is harmless, and excess is easily
eliminated. So eat all the carrots you want! You might turn orange,
but it probably won't kill you.

Butterflies
Yesterday I visited the American Museum of Natural History in New York
City, for the first time in many years. They have a special exhibit
of butterflies. They get pupae shipped in from farms, and pin the
pupae to wooden racks; when the adults emerge, they get to flutter
around in a heated room that is furnished with plants, ponds of
nectar, and cut fruit.

The really interesting thing I learned was that chrysalises are not
featureless lumps. You can see something of the shape of the animal
in them. (See, for example, this
Wikipedia illustration.) The caterpillar has an exoskeleton,
which it molts several times as it grows. When time comes to pupate,
the chrysalis is in fact the final exoskeleton, part of the animal
itself. This is in contrast to a cocoon, which is different. A
cocoon is a case made of silk or leaves that is not part of the
animal; the animal builds it and lives inside. When you think of a
featureless round lump, you're thinking of a cocoon.

Until recently, I had the idea that the larva's legs get longer, wings
sprout, and so forth, but it's not like that at all. Instead, inside
the chrysalis, almost the entire animal breaks down into a liquid!
The metamorphosis then reorganizes this soup into an adult. I asked
the explainer at the Museum if the individual cells retained their
identities, or if they were broken down into component chemicals. She
didn't know, unfortunately. I hope to find this out in coming weeks.

How does the animal reorganize itself during metamorphosis? How does
its body know what new shape to grow into? It's all a big mystery.
It's nice that we still have big mysteries. Not all mysteries have
survived the scientific revolution. What makes the rain fall and the
lightning strike? Solved problems. What happens to the food we eat,
and why do we breathe? Well-understood. How does the butterfly
reorganize itself from caterpillar soup? It's a big puzzle.

A related puzzle is how a single cell turns into a human baby during
gestation. For a while, the thing doubles, then doubles again, and
again, becoming roughly spherical, as you'd expect. But then stuff
starts to happen: it dimples, and folds over; three layers form, a
miracle occurs, and eventually you get a small but perfectly-formed
human being. How do the cells in the fingers decide to turn into
fingers? How does the cells in the fourth finger know they're one
finger from one side of the hand and three fingers from the other
side? Maybe the formation of the adult insect inside the chrysalis
uses a similar mechanism. Or maybe it's completely different. Both
possibilities are mind-boggling.

This is nowhere near being the biggest pending mystery; I think we at
least have some idea of where to start looking for the answer.
Contrast this with the question of how it is we are conscious, where
nobody even has a good idea of what the question is.

Other caterpillar news: chrysalides are so named because they often
have a bright golden sheen, or golden features. (Greek "khrusos" is
"gold".) The
Wikipedia picture of this is excellent too. The "gold" is a
yellow pigmented area covered with a shiny coating. The explainer
said that some people speculate that it helps break up the outlines of
the pupa and camouflage it.

I asked if the chrysalis of the viceroy butterfly, which, as an adult,
resembles the poisonous monarch butterfly, also resembled the
monarch's chrysalis. The answer: no, they look completely different.
Isn't that interesting? You'd think that the pupa would get at least
as much benefit from mimicry as the adult. One possible explanation
why not: most pupae don't make it to adulthood anyway, so the marginal
benefit to the species from mimicry in the pupal stage is small
compared with the benefit in the adult stage. Another: the pupa's
main defense, which is not available to the adult, is to be difficult
to see; beyond that it doesn't matter much what happens if it
is seen. Which is correct? I don't know.

For a long time folks thought that the monarch was poisonous and the
viceroy was not, and that the viceroy's monarch-like coloring tricked
predators into avoiding it unnecessarily. It's now believed that
both speciies are poisonous and bad-tasting, and that their
similar coloring therefore protects both species. A predator who eats
one will avoid both in the future. The former kind of mimicry is
called Batesian; the latter, Müllerian.

The monarch butterfly does not manufacture its toxic and bad-tasting
chemicals itself. It is poisonous because it ingests poisonous
chemicals in its food, which I think is milkweed plants. Plant
chemistry is very weird. Think of all the poisonous foods you've ever
heard of. Very few of them are animals. (The only poisonous meat I
can think of offhand is the liver of arctic animals, which has a
toxically high concentration of vitamin D.) If you're stuck on a
desert island, you're a lot safer eating strange animals than you are
eating strange berries.

Daniel
Dennett is a philosopher of mind and consciousness. The first
work of his that came to my attention was his essay "Why You Can't
Build a Computer That Can Feel Pain". This is just the sort of topic
that college sophomores love to argue about at midnight in the dorm
lounge, the kind of argument that drives me away, screaming "Shut up!
Shut up! Shut up!"

But to my lasting surprise, this essay really had something to say.
Dennett marshaled an impressive amount of factual evidence for his
point of view, and found arguments that I wouldn't have thought of.
At the end, I felt as though I really knew something about this topic,
whereas before I read the essay, I wouldn't have imagined that there
was anything to know about it. Since then, I've tried hard to
read everything I can find that Dennett has written.

I highly recommend Dennett's 1995 book Darwin's Dangerous
Idea. It's a long book, and it's not the main point of my
essay today. I want to give you some sense of what it's about,
without straining myself to write a complete review. Like all really
good books, it has several intertwined themes, and my quoting can only
expose part of one of them:

A teleological explanation is one the explains the existence or
occurrence of something by citing a goal or purpose that is served by
the thing. Artifacts are the most obvious cases; the goal or purpose
of an artifact is the function it was designed to serve by its
creator. There is no controversy about the telos of a hammer:
it is for hammering in and pulling out nails. The telos of
more complicated artifacts, such as camcorders or tow trucks or CT
scanners, is if anything more obvious. But even in simple cases, a
problem can be seen to loom in the background:

"Why are you sawing that board?"
"To make a door."
"And what is the door for?"
"To secure my house."
"And why do you want a secure house?"
"So I can sleep nights."
"And why do you want to sleep nights?"
"Go run along and stop asking such silly questions."

This exchange reveals one of the troubles with teleology: where does
it all stop? What final cause can be cited to bring this
hierarchy of reasons to a close? Aristotle had an answer: God, the
Prime Mover, the for-which to end all for-whiches. The
idea, which is taken up by the Christian, Jewish, and Islamic
traditions, is that all our purposes are ultimately God's
purposes. . . . But what are God's purposes? That is something of a
mystery.

. . . One of Darwin's fundamental contributions is showing us a new
way to make sense of "why" questions. Like it or not, Darwin's idea
offers one way—a clear, cogent, surprisingly versatile way—of
dissolving these old conundrums. It takes some getting used to, and
is often misapplied, even by its staunchest friends. Gradually
exposing and clarifying this way of thinking is a central project of
the present book. Darwinian thinking must be carefully distinguished
from some oversimplified and all-too-popular impostors, and this will
take us into some technicalities, but it is worth it. The prize is,
for the first time, a stable system of explanation that does not go
round and round in circles or spiral off in an infinite regress of
mysteries. Some people would very much prefer the infinite regress of
mysteries, apparently, but in this day and age the cost is
prohibitive: you have to get yourself deceived. You can either
deceive yourself or let others do the dirty work, but there is no
intellectually defensible way of rebuilding the mighty barriers to
comprehension that Darwin smashed.

(Darwin's Dangerous Idea, pp. 24–25.)

Anyway, there's one place in this otherwise excellent book where
Dennett really blew it. First he quotes from a 1988 Boston
Globe article by Chet Raymo, "Mysterious Sleep":

University of Chicago sleep researcher Allan Rechtshaffen asks "how
could natural selection with its irrevocable logic have 'permitted'
the animal kingdom to pay the price of sleep for no good reason?
Sleep is so apparently maladaptive that it is hard to understand why
some other condition did not evolve to satisfy whatever need it is
that sleep satisfies.

And then Dennett argues:

But why does sleep need a "clear biological function" at all? It is
being awake that needs an explanation, and presumably its
explanation is obvious. Animals—unlike plants—need to be awake at
least part of the time in order to search for food and procreate, as
Raymo notes. But once you've headed down this path of leading an
active existence, the cost-benefit analysis of the options that arise
is far from obvious. Being awake is relatively costly, compared with
lying dormant. So presumably Mother Nature economizes where she
can. . . . But surely we animals are at greater risk from predators
while we sleep? Not necessarily. Leaving the den is risky, too, and
if we're going to minimize that risky phase, we might as well keep the
metabolism idling while we bide our time, conserving energy for the
main business of replicating.

This is a terrible argument, because Dennett has apparently missed the
really interesting question here. The question isn't why we sleep;
it's why we need to sleep. Let's consider another important
function, eating. There's no question about why we eat. We eat
because we need to eat, and there's no question about why we
need to eat either. Sure, eating might be maladaptive: you have to
leave the den and expose yourself to danger. It would be very
convenient not to have to eat. But just as clearly, not eating won't
work, because you need to eat. You have to get energy from somewhere;
you simply cannot run your physiology without eating something once in
a while. Fine.

But suppose you are in your den, and you are hungry, and need to go
out to find food. But there is a predator sniffing around the door,
waiting for you. You have a choice: you can stay in and go hungry,
using up the reserves that were stored either in your body or in your
den. When you run out of food, you can still go without, even though
the consequences to your health of this choice may be terrible. In
the final extremity, you have the option of starving to death, and
that might, under certain circumstances, be a better strategy than
going out to be immediately mauled by the predator.

With sleep, you have no such options. If you're treed by a panther,
and you need to stay awake to balance on your branch, you have no
options. You cannot use up your stored reserves of sleep. You do not
have the option to go without sleep in the hope that the panther will
get bored and depart. You cannot postpone sleep and suffer the
physical consequences. You cannot choose to die from lack of sleep
rather than give up and fall out of the tree. Sooner or later you
will sleep, whether you choose to or not, and when you sleep you will
fall out of the tree and die.

People can and do go on hunger strikes, refuse to eat, and starve to
death. Nobody goes on sleep strikes. They can't. Why not? Because
they can't. But why can't they? I don't think anyone
knows.

The question isn't about the maladaptivity of sleeping itself; it's
about the maladaptivity of being unable to prevent or even to
delay sleep. Sleep is not merely a strategy to keep us
conveniently out of trouble. If that were all it was, we would need
to sleep only when it was safe, and we would be able to forgo it when
we were in trouble. Sleep, even more than food, must serve some vital
physiological role. The role must be so essential that it is
impossible to run a mammalian physiology without it, even for as long
as three days. Otherwise, there would be adaptive value in being able
to postpone sleep for three days, rather than to fall asleep
involuntarily and be at the mercy of one's enemies.

Given that, it is indeed a puzzle that we have not been able to
identify the vital physiological role of sleep, and Rechtshaffen's
puzzlement above makes sense.